Across the globe there is great demand for disposable paper products such as sanitary tissue and facial tissue. In the North American market, the demand is increasing for higher quality products offered at a reasonable price point. The quality attributes most important for consumers of disposable sanitary tissue and facial tissue are softness and strength.
Softness is the pleasing tactile sensation the consumers perceive when using the tissue product as it is moved across his or her skin or crumpled in his or her hand. The tissue physical attributes which affect softness are primarily surface smoothness and bulk structure.
The surface smoothness is primarily a function of the surface topography of the web. The surface topography is influenced by the manufacturing method such as conventional dry crepe, through air drying (TAD), or hybrid technologies such as Metso's NTT, Georgia Pacific's ETAD, or Voith's ATMOS process. The manufacturing method of conventional dry crepe creates a surface topography that is primarily influenced by the creping process (doctoring a flat, pressed sheet off of a steam pressurized drying cylinder) versus TAD and hybrid technologies which create a web whose surface topography is influenced primarily by the structured fabric pattern that is imprinted into the sheet and secondarily influenced by the degree of fabric crepe and conventional creping utilized. A structured fabric consists of monofilament polymeric fibers with a weave pattern that creates raised knuckles and depressed valleys to allow for a web with high Z-direction thickness and unique surface topography. Thus, the design of the structured fabric is essential in controlling the softness and quality attributes of the web. U.S. Pat. No. 3,301,746 discloses the first structured or imprinting fabric designed for production of tissue. A structured fabric may also contain an overlaid hardened photosensitive resin to create a unique surface topography and bulk structure as shown in U.S. Pat. No. 4,529,480.
Fabric crepe is the process of using speed differential between a forming and structured fabric to facilitate filling the valleys of the structured fabric with fiber, and folding the web in the Z-direction to create thickness and influence surface topography. Conventional creping is the use of a doctor blade to remove a web that is adhered to a steam heated cylinder, coated with an adhesive chemistry, in conjunction with speed differential between the Yankee dryer and reel drum to fold the web in the Z-direction to create thickness, drape, and to influence the surface topography of the web. The process of calendering, pressing the web between cylinders, will also affect surface topography. The surface topography can also be influenced by the coarseness and stiffness of the fibers used in the web, degree of fiber refining, as well as embossing in the converting process. Added chemical softeners and lotions can also affect the perception of smoothness by creating a lubricious surface coating that reduces friction between the web and the skin of the consumer.
The bulk structure of the web is influenced primarily by web thickness and flexibility (or drape). TAD and Hybrid Technologies have the ability to create a thicker web since structured fabrics, fabric crepe, and conventional creping can be utilized while conventional dry crepe can only utilize conventional creping, and to a lesser extent basis weight/grammage, to influence web thickness. The increase in thickness of the web through embossing does not improve softness since the thickness comes by compacting sections of the web and pushing these sections out of the plane of the web. Plying two or more webs together in the converting process, to increase the finished product thickness, is also an effective method to improve bulk structure softness.
The flexibility, or drape, of the web is primarily affected by the overall web strength and structure. Strength is the ability of a paper web to retain its physical integrity during use and is primarily affected by the degree of cellulose fiber to fiber hydrogen bonding, and ionic and covalent bonding between the cellulose fibers and polymers added to the web. The stiffness of the fibers themselves, along with the degree of fabric and conventional crepe utilized, and the process of embossing will also influence the flexibility of the web. The structure of the sheet, or orientation of the fibers in all three dimensions, is primarily affected by the manufacturing method used.